Electroluminescent organic material has become an intensely adapted research field since its first observation in 1953 (Bernanose et al., J. Chem. Phys. 1953, 50, 65). The known advantages of organic materials for generating light—such as minimal re-absorption, high quantum efficiency (QE), or the possibility to adapt the emissions spectrum through relatively simple variation of the molecule structure, for example—were able to be exploited in recent years, through constant development in material research and the implementation of new concepts, for effective injection and transportation of charge-carriers into the active emissions layer of an organic light element. The first display devices, which were based on such so-named organic LEDs, have already found their way onto the market, and organic LEDs will, in the future, as a concept be firmly established next to liquid crystal displays (LCDs) and displays of inorganic LEDs. Another market which stands open to the organic LEDs, due to their special ability to emit light in a widespread and homogeneous way into half space, is the area of illumination.
Light is often emitted through a transparent substrate (the so-called bottom-emission structure) in organic LEDs. In some cases, it can be advantageous to use a non-transparent substrate for improving the performance parameters of the component or for reducing the production costs. In this case, the light is emitted through the top electrode of the layer structure. For this purpose, a sufficiently transparent top electrode is needed. Both approaches can also be combined in order to obtain transparent organic LEDs.
Transparent top electrodes are traditionally made of transparent conductive metal oxides, particularly Indium-Tin-Oxide (ITO). This approach has several difficulties, which compromise the operational capability of this solution. The conductivity of the ITO layer is low, so that in widespread structures, a voltage drop in the layer and therewith a lateral inhomogeneous light emission results from the sheet resistance. Furthermore, by means of a sputtering process, the layer or film is deposited. The underlying organic layers are usually very susceptible to the reactive ions which arise by this process. Adverse effects for the performance parameters of the component and the operational life span can only be party compensated by means of inserts of buffering layers. Usually, the metal oxides are well-suited for the injection of holes, namely for application as anodes, while the injection of electrons causes problems. Finally, the high proportion of indium in the ITO film means a significant cost factor.
Alternatively, these metal films or layers can be used as transparent top electrodes. Thin metal layers like 15 nm silver, for example, exhibit only minimal absorption and are sufficiently transparent. Adverse is the difficulty to controlledly deposit such thin layers. Furthermore these layers are not mechanically stable and often lead to a premature failure of the component due to a tear or crack in the top electrode. Eventually, the high reflectivity of the top electrode leads to difficulties controlling the interference effects within the component. The high Q-factor of the resulting micro-cavity results in the preferment of a narrower wavelength range of the resulting light, while other ranges are being suppressed. Particularly when white light is emitted, this results in a distortion of the intended emissions spectrum. So this component cannot be utilized for some application fields—for example, as illuminating element or as background illuminating element of an LCD display.
Eventually, it was proposed in the document US 2004/0021434 A1 that a top electrode with passages be provided. It became apparent that such passages adversely affect the shining impression of the component, since non-shining sections appeared in their range.